The present invention is particularly applicable for use in a passive seat belt restraint system for a motor vehicle of the type employing a seat belt retractor located in a door of the vehicle and will be described with particular reference to this type of system; however, it is appreciated that the invention has much broader applications and may be employed in other seat belt retractor environments wherein there is a desire or need to modify the inertia forces necessary for locking the retractor from paying out a seat belt during rapid deceleration or acceleration of the vehicle.
In the automobile industry, there has been a long term development effort in providing passenger protection during times of collision and other emergency situations. At first, seat belts were used across the lap of vehicle occupants in a system requiring the occupant to physically attach the seat belt. This type of system was referred to as "active" in that it required the participation of the vehicle occupant. Many of these systems were developed and employed over a number of years in automobiles and other motor vehicles. These systems often employed a seat belt retractor including a spool on which the excess seat belt would be reeled for snug engagement with the occupants. These reels or seat belt retractor mechanisms ultimately were equipped with inertia sensing mechanisms or elements whereby the retractor mechanism would lock whenever forces exceeding a preselected limit, such as 0.5 g, was experienced by the vehicle. Consequently, a forward collision or rear impact would lock the seat belt retractor mechanism to restrain the occupants during emergency situations. During normal operation, the seat belt could be easily unreeled to the desired comfortable position on passengers of different statures. These inertia locking devices for the seat belt retractor mechanisms generally included either a tripping or swinging weight, the first of which was known as a "standing man" and the other a "pendulum". Inertia would either trip or swing the inertia sensing device and this action would result in the seat belt retractor mechanism being locked. These active seat belt systems were quite successful and could be used in either a lap belt system, a shoulder harness system or a combination thereof; however, they all were active systems requiring the definite intervention of the occupant before protection was obtained.
After substantial development work, one of the more popular retractor mechanisms of the type having an inertia sensing arrangement involved a rotatable spool having a ratchet plate at one end and the inertia sensor element was a pendulum device engageable with a locking bar to drive the locking bar into the ratchet plate upon rapid acceleration or deceleration of the vehicle. To add strength to this arrangement, two separate ratchet plates were located at opposite ends of the spool and were engageable with the same locking bar. This system was mechanically sound and obtained the desired results demanded by the rigor of the environment in which it was to be operated.
The last several years has found an increase in the desire for some type of restraint system for motor vehicles that would be "passive" in nature in that the occupants of a vehicle would be automatically restrained without their manipulative intervention. One passive system which is now being considered for use on motor vehicles is a system wherein the one or two belts are attached on the inboard side of the front seats of a motor vehicle and the seat belt retractors for the belts are located in the vehicle door so that as the door is opened the occupants enter the vehicle behind the seat belt or belts. Upon closing the door, the belts surround the occupants and a retractor mechanism within the door for one or two belts retracts the belt or belts into the operative position. Seat belt retractors in the door of the motor vehicle and equipped with the necessary inertia locking devices have heretofore presented certain technical problems. As someone opens the door of the vehicle, the reel must freely payout so that the door can open; however, should the opening action of the door exceed the sensed inertia force of the retractor mechanisms, the mechanisms would be locked. The door could not be opened further. For that reason, seat belt retractors located in the door of motor vehicles and including inertia sensing mechanisms for locking the mechanisms have been provided with a variety of arrangements for providing normal operation of the inertia sensing elements except when the door was open. At first, it was suggested to use mechanical lockouts or solenoid operated lever systems wherein, as the door is opened, the inertia device becomes inactive. This did solve the problem of opening the door, unless there was mechanical failure. However, some accidents occur with the door open, which would discourage use of such mechanical lockouts. To overcome the complex problems resulting from placing inertia locked seat belt retractors in vehicle doors, it has been suggested that various arrangements could be employed for increasing slightly the inertia force necessary for activating the locking arrangement of the retractor mechanism when the door is opened. This increase was to a degree that the mechanism would be responsive to collisions and other abrupt changes in the vehicle speed even when the door was opened. In other words, it was suggested that the movement of the inertia sensor element could be restrained slightly when the door is opened, but not completely locked from operation as suggested in the past. Several arrangements for increasing the force of the inertia sensor in a door mounted retractor mechanism are disclosed in Hollowell U.S. Pat. No. 4,181,326, which is incorporated by reference herein as background information. In accordance with this particular patent, an electromagnet can be employed for increasing the force of the swinging pendulum. Further, a permanent magnet can be employed on the standard pivoted locking bar for applying a certain amount of increased resistance to pivoting the bar into the locking position when the door is opened. Sensing of the door opened condition can be accomplished by a standard doorjamb switch that is closed when the door is opened. The most successful approach shown in the Hollowell patent is the concept of employing an electromagnet coacting with the pendulum to increase the inertia force necessary to move the pendulum when the door is in the opened position. The same concept is illustrated in de Rosa U.S. Pat. No. 4,007,802 on a spool having a single ratchet plate on one end.
A further arrangement for employing a permanent magnet to increase the inertia force necessary to trip a door mounted retractor mechanism of the type contemplated by the present invention is illustrated in Collins U.S. Pat. No. 4,235,455. The de Rosa and Collins patents are also incorporated by reference herein for the purposes of background information. As can be seen from the prior art structures illustrated in these patents, it is known to employ a permanent magnet operating on the bottom of a pendulum or a permanent magnet operating on a portion of the pivoted lock bar both for the purpose of increasing slightly the inertia force necessary for tripping the lock bar when the door is opened as indicated by the position of a doorjamb switch.
Seat belt retractor mechanisms as illustrated in the prior art and described above had substantial disadvantages. When mechanical overriding arrangements were employed, the moving parts could wear out and could conceivably hold the locking bar in the inactive position. When solenoids were used to operate linkage systems directly, a movable core member was employed. This core member could lock in various positions, especially when subjected to adverse environmental conditions. Moving parts would present difficulties as would various assembly tolerance accumulations. Also, solenoids would require substantial currents and would present electrical drains on the battery if locked into certain positions. Such solenoids are shown in the Hollowell patent. To overcome the disadvantages of this solenoid arrangement, the electromagnet concept with a fixed core has been suggested as in all three patents incorporated by reference herein. By utilizing an electromagnet concept, disadvantages of the solenoid have been overcome; however, electromagnets, as suggested by the prior art patents described above, have certain disadvantages the most prevalent of which was the susceptibility of the magnetic force to manufacturing tolerances. Since the magnetic force in a magnetic circuit is highly dependent upon mechanical tolerances and electrical parameters, such permanent magnets are susceptible to complex geometrical and electrical variations generally unacceptable from a cost standpoint and operational predictability needed for wide spread use in mass production of motor vehicles.